This invention relates to fuel cells and its used in connection with gas turbines, steam turbines, and heating, ventilation and air conditioning (HVAC) systems, and specifically to high performance hybrid power systems employing such devices.
Conventional high performance gas turbine power systems exist and are known. Prior gas turbine power systems include a compressor, a combustor, and a mechanical turbine, typically connected in-line, e.g., connected along the same axis. In a conventional gas turbine, air enters the compressor and exits at a desirable elevated pressure. This high-pressure air stream enters the combustor, where it reacts with fuel, and is heated to a selected elevated temperature. This heated gas stream then enters the gas turbine and expands adiabatically, thereby performing work. One deficiency of gas turbines of this general type is that the turbine typically operates at relatively low system efficiencies, for example, around 25%, with systems of megawatt capacity.
One prior art method employed to overcome this problem is to employ a recuperator for recovering heat. This recovered heat is typically used to further heat the air stream prior to the stream entering the combustor. Typically, the recuperator improves the system efficiency of the gas turbine upwards to about 30%. A drawback of this solution is that the recuperator is relatively expensive and thus greatly adds to the overall cost of the power system.
Another prior art method employed is to operate the system at a relatively high pressure and a relatively high temperature to thereby increase system efficiency. However, the actual increase in system efficiency has been nominal, while the system is subjected to the costs associated with the high temperature and pressure mechanical components.
Still another prior art method utilized by plants having power capacities above 100 MW is to thermally couple the high temperature exhaust of the turbine with a heat recovery steam generator for a combined gas turbine/steam turbine application. This combined cycle application typically improves the system operating efficiency upwards to about 55%. However, this efficiency is still relatively low.
The overall power system performance is further predicated on the efficiency of the constituent fuel cells and associated cooling systems. The traditional method for fuel cell thermal management is to force high volumes of a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Cooling water is often employed for ambient temperature devices, and air can be employed for higher temperature fuel cells. In some instances, the same air which serves as the fuel cell's oxidant is used as a cooling medium as well. The cooling medium passes through the fuel cell and carries off the thermal energy by its sensible heat capacity. The volume flow of coolant required for this method is inversely related to the limited temperature operating range of the electrochemical operation of the electrolyte, or in the case of fuel cells with ceramic components, by constraints associated with thermal stress.
The foregoing heat capacity limitations on the amount of temperature rise of the cooling medium result in coolant flow rates through the fuel cell much higher than those required by the electrochemical reaction alone. Since these relatively large flow quantities must be preheated to a temperature at or near the operating temperature of the fuel cell and circulated therethrough, a dedicated reactant thermal management subsystem is required. Typically, the coolant is preheated to a temperature either at or near the fuel cell operating temperature, e.g., within 50.degree. C. of the operating temperature. Such thermal management subsystems normally include equipment for regenerative heating, pumping, and processing of the excessive coolant flow. These components add substantially to the overall cost of the system.
For illustration purposes, consider a regenerative heat exchanger of a type suitable for preheating the fuel cell reactants and operating with a 100.degree. C. temperature difference, and a typical heat transfer rate of 500 Btu/hr-ft.sup.2 (0.13 W/cm.sup.2). Further assuming a 50% cell efficiency with no excess coolant flow, and operating at an ambient pressure, the heat processing or heat transfer surface area of the regenerator would be of the same order of magnitude as the surface area of the fuel cell electrolyte. Considering an excess coolant flow requirement of 10 times the level required for the fuel cell reactant flow, a representative value for conventional approaches, the heat exchanger surface area would be 10 times larger than the active fuel cell surface area. The large size of this heat exchanger makes it difficult to integrate the heat exchanger with electrochemical converters to form a compact and efficient power system.
Furthermore, the high volume of cooling fluids being passed through the fuel cell makes the fuel cell unsuitable for direct integration with the gas turbine to achieve relatively high system efficiency.
Thus, there exists a need in the art for high performance power systems and for systems that provide for better thermal management approaches, especially for use in electrochemical or hybrid power energy systems. In particular, an improved power system, such as a gas turbine power system, that is capable of integrating and employing the desirable properties of electrochemical converters would represent a major improvement in the industry. More particularly, an integrated electrochemical converter assembly for use with a gas turbine system that reduces the costs associated with providing effective thermal processing approaches while significantly increasing the overall system power efficiency, would also represent a major improvement in the art.